130 SUPERPHOSPHATES SUPERPHOSPHATES In these products the phosphoric acid occurs mostly us mono calcium phosphate, CatHaPO^a, and to a less extent as dicalcium phosphate, CaHP04, that is, in two forms soluble in water and in ammonium citrate ; smaller quantities may occur as tncaldum phosphate, insoluble, in water or the citrate, but soluble in acids. Superphosphates contain also free phosphoric and sulphuric acids, calcium sulphate, silica, and other extraneous substances, according to their origin (see later). Superphosphates are distinguished as bone- superphosphates ; mineral superphosphates ; and double, triple, and enriched, super phosphates, which are obtained by treating the phosphates with phosphoric acid. The analysis of these products includes the following : 1. Moisture.—Sec- General Methods, 2. 2, Phosphoric Anhydride.- That soluble in water and ammonium citrate, and that soluble in water alone, are. determined. A, PHOSPHORIC ANHYDKIDK sor.UHi.i-; IN WATKK AND IN AMMONIUM CITRATE (Appiani's method}. This requires the solutions (ammonium citrate, magnesia mixture1 and ammonia) prescribed lor the determination of the total phosphoric acid (see General Methods, 4) : 5 grams of the super- phosphate (2-5 grams with double or triple superphosphate) are made into a paste; in a mortar with 4.0 50 c,c. of water, allowed to si ami a few minutes and the liquid decanted on to a pleated filter, the filtrate being collected in • a 250 e,e. measuring lla.sk. This treatment with water is repeated three or four times, the operations being regulated so 11 nil the digestion lasts only a few minutes and the. filter is always empty when the decanted liquid is placed in it. The whole of the solid matter is .finally washed on to the filter and there washed unlil the volume of the. total nitrate occupies nearly 250 c.e.. ; a few drops of hydrochloric, or nitric acid are then added and Ilie volume, made up with water (aqueous solution], The alter and its contents an- introduced into another 250 c.c. measuring flask and digested with 100 c.c. of the ammonium citrate solution l for an hour a,t 35 40", with frequent shaking; when cool, Hie volume is rnatU; up to 250 c.c. with water and the liquid shaken and filtered (citric solution). To 50 c.c, of the aqueous solution are added 50 c.c.. of the citric solution, 50 c.c. of water, 50 c.c. of ammonia, (1) o-tju) and then, gradually and with shaking, 50 c.c. of magnesia, mixture. Alter the whole has been well shaken, 1 This amount of citrate is usually more than lutllu tent lu dissolve all Uu- dkaldum phosphate in tin- residue insoluble in water from ordinary < ummercial superphosphates, inwhii'li form I'our-lilihs in nine ten 1 ItsoI' Ihr phosphottc at id soluble in citrate, is soluble in water. With precipitated phosphates, with superphosphates of high Brittle, but poor in phosphoric acid soluble in water, and with phosphates quite free from mommal- dura phosphate, so that part of the phosphate init;hl remain nndi'<solved, it; is advisable to nse more citrate or to work with a .smaller quantity ol ''Ubstauee, With double and triple superphosphates, meat mianu, or excessively dry super- phosphates, in order to include ;I]SM any pvro- and m«'ta phuHphnrie acids present it is twlvisiible, before precipitating the jihoivphorir ucid. t»> he,1,1 the s«»lut.um for some time with a little nitric, add to tranhfonn into phosphoric at id the pyrti ami meta-pho.Hplwric acids wliich may be fonwul during drying.